The Invention relates to a continuous boiling apparatus the kind for organic materials, e.g. fish or offal from albattoirs.
Such continuous boiling apparatus is known in various con-figurations and is arranged for heat treatment, coagulation or boiling of raw organic materials, e.g., raw fish or offal from abattoirs and the like, which is boiled before further processing in the manufacture of various products such as fish oil, fishmeal, meat-and-bone meal, blood meal etc.
A commonly-known apparatus, for example the applicants' own "Continuous Indirect Boiler", consists of a horizontally-lying worm conveyor surrounded by a jacket, where both the jacket and the worm conveyor are heated with steam. When desired to have a large capacity, apparatus of this kind becomes very voluminous. For example, a boiling apparatus with a capacity of 27 tons of fish per hour will have a length of approx. 14 m, a height of approx. 1.8 m, a breadth of approx. 1.5 m and a weight of approx. 16 tons. To drive the worm conveyor, quite a large electromotor is required with a power consumption of approx. 11 kW. Apparatus of this kind is voluminous but, however, the greatest disadvantage is that the raw material has a great tendency to leave deposits on the inside heating surfaces due to the fast burning of the organic material. During operation, considerable deposits are left on both the jacket and the worm conveyor. Such deposits are highly undesirable, the reason being that the capacity is drastically reduced. Normally, the deposits are removed in connection with the stopping of the apparatus and subsequent cleaning by soaking with lye or the like and/or high-pressure sluicing, i.e. methods which are extremely time-consuming and which reduce the total capacity of the apparatus, the reason being that such cleaning takes a long time and must be carried out at relatively frequent intervals. With the object of extending the operation time between cleaning, it is known during operation to use a direct injection of steam into the boiling zone. However, there is the great disadvantage that the raw material is attenuated and gets mixed with the condensating steam, the result being that extra energy and resources must be used to remove the condensate.
The use of a boiler with a worm conveyor often results in an uneven boiling of the raw material, in that the raw material is not stirred while being fed forward but merely pushed forwards by the worm thread, and is boiled or heat-treated by the heat infiltrating the raw material from both sides. Consequently, if it is to be ensured that all the raw material has been completely boiled or heat-treated as desired, this demands a relatively long period of time in such a boiling apparatus. At the same time, a part of the raw material will be over-boiled.
Norwegian published application no. 134,727 describes a vertically-positioned, tubular boiler for the heat treatment of proteinaceous fish material where the product mass is heated by heating plates for steam and by introducing steam directly and contrary to the flow of the product mass. The latter has the great disadvantage that the product mass is attenuated by condensate which must be removed afterwards in an energy-consuming process. The boiler has combined scrapers and stirring means lifting the product mass along the wall of the boiler contrary to the direction of transport and thereby contrary to gravitation, this being highly energy-consuming. The boiler is closed off from the surroundings and the raw material is introduced through a worm conveyor. Consequently, the raw material must be comminuted before being introduced. Thus, the boiler consumes a relatively large amount of energy to ensure stirring and that the wall of the boiler is scraped clean. It will moreover be necessary at intervals to clean the large number of threaded stirring means and the stationary steam induction nozzles etc., which requires that the boiler is stopped and emptied at intervals.
The company Alfa-Laval has produced another type of vertical boiler under the name "Contherm" for the boiling of fish pulp, chopped and comminuted offal from abattoirs etc. In order to avoid burning-in on the heating surfaces consisting of a steam-heated jacket, the boiling chamber has internal scrapers for continuous cleaning. The boiler requires that the raw material is comminuted and also consumes a large amount of energy for the scrapers. The boiler is closed and under pressure as the material is fed forward by means of the pressure difference between a bottom inlet and a top outlet, i.e. contrary to gravitation. As an example, a boiler with a capacity of only 1.1 tons of comminuted raw material per hour requires a drive motor of 7.5 kW for the scrapers.